In recent years, there has been a growing demand for higher quality of metal materials. Relatively large inclusion possibly generated in the process of deoxidation of steel or the like is causative of considerable degradation in steel quality. For example, aluminum-series oxides may induce various non-conformities such as surface defect on a thin automotive steel sheet, crack in a process of manufacturing a beverage can, breaking of wire products in a process of drawing, and so forth. Numerous efforts have, therefore, been made aiming at reducing the amount and size of such inclusions.
On the other hand, also numerous efforts have been made in order to intentionally increase or decrease the amount and size of fine inclusions, so as to further improve quality of steel. For example, efforts have been made on allowing a significant amount of fine precipitates to precipitate in steel, or downsizing grain size of steel, during various processes of hot rolling, cold rolling, and heat treatment such as continuous annealing, stress relief annealing, welding and so forth, to thereby improve strength and toughness of welded portion. Efforts have been made still also on reducing the amount of fine precipitates, and increasing coarse grains, to thereby improve the iron loss.
Accordingly, for the purpose of highly-reproducible mass production of high-quality steel on the industrial basis, conventional analytical values of component alone may be insufficient to fully understand the amount and size of particles contained in steel, so that it is important to develop a method of analyzing particle size distribution of particles in steel, capable of validating the amount and size of the particles in a correct and highly reproducible manner.
Conventionally known methods of inspecting particles in steel include microscopic inspection methods such as ASTM method, JIS method, and a MICHELIN method developed by MICHELIN. For example, a microscopic inspection method specified by Non-Patent Document 1 is such as polishing a metal sample, and observing the metal sample under a microscope at least in 60 or more field of views at a 400× magnification in principle, so as to judge the degree of cleanliness of steel based on the ratio of area occupied by particles such as inclusion. All of these conventional methods rely upon visual inspection under an optical microscope, and therefore suffer from slow speed of inspection. Another problem is that the methods suffer from large error, and are therefore difficult to ensure highly accurate measurement, because there is no obvious rule for discriminating the inclusions from misconceptional factors such as dust, polishing defect, rust and so forth.
Patent Document 1 describes a method aimed at making up for low number density in this sort of microscopic observation. In this conventional method, steel is electrolyzed first, and the extracted residue is then dropped onto a support film and allowed to dry, to thereby produce a sample having an extremely large number of residue particles. The sample is then subjected to optical microscopic analysis, scanning electron microscopic (SEM) analysis, transmission electron microscopic (TEM) analysis and so forth. Patent Document 1 also describes that an effect of this sort of method is such that a highly representative particle size distribution data contributed by a large number of particles may be obtained.
However, in this method, the sample contains large particles and small particles in a mixed manner. Accordingly, in order to measure a distribution over the entire sizes from the individual photos in the microscopic analysis, a large number of times of photographing and image processing, and even a manual counting by an operator, may be necessary. It may, therefore, be impossible to improve the speed of inspection, and may be difficult to obtain good reproducibility due to a large tendency of causing individual difference.
Another method of evaluation different from the microscopic inspection is described in Patent Document 2 and Non-Patent Document 2. According to the method of evaluation, a metal sample is subjected to optical emission spectrometry under spark discharge induced by approximately 2,000 pulses, in which particle size of oxide is determined based on discharge data exclusive of a preliminary discharge data corresponded to initial several hundred pulses. In this method of evaluation, a very intense optical emission (abnormal emission) ascribable to constitutive element of the oxide is assumed as being derived from a single oxide particle.
Still another method of determining the size and frequency of alumina inclusion is described in Non-Patent Document 3. In this method, a metal sample is subjected to optical emission spectrometry under spark discharge, and the size and frequency of alumina inclusion are determined based on intensity of optical emission spectrometric data, while assuming that only pulse data exceeding a predetermined threshold value is ascribable to the inclusion and so forth.
These methods, relying upon optical data processing of optical information called optical emission intensity, are less causative of individual difference, and therefore advantageously enable compositional analyses making use of simultaneous optical emission by multiple elements.
However, these methods are not considered to be highly accurate, since the hypotheses described in Patent Document 2 and Non-Patent Document 2 are incorrect. More specifically, so far as actual traces of discharge of several millimeters in diameter are observed, it may be natural to suppose that “a single time of pulse emission is ascribable to a plurality of particles of inclusion (oxide)”, so that the hypothesis described in the above are not correct.
In addition, since the particles such as inclusion contributive to the optical emission in these methods of optical emission spectrometry are larger than several micrometers in principle, so that pulse intensity of the particles cannot be compared with that of the solid-solubilized components in the matrix, unless the particles have such large sizes. In other words, the optical emission spectrometry is not adoptable to particles having sizes smaller than several micrometers, and this makes the analyses incorrect.
As has been described in the above, it is very important to quantitatively analyze the size, frequency and composition of particles in metal in a rapid and correct manner, in view of quality control of metal materials. This is, however, not attainable by the prior arts.